Access signal adjustment circuits and methods for memory cells in a cross-point array

ABSTRACT

Embodiments of the invention relate generally to semiconductors and memory technology, and more particularly, to systems, integrated circuits, and methods to generate access signals to facilitate memory operations in scaled arrays of memory elements, such as memory implemented in third dimensional memory technology formed BEOL directly on top of a FEOL substrate that includes data access circuitry. In at least some embodiments, a non-volatile memory device can include a cross-point array having resistive memory elements disposed among word lines and subsets of bit lines, and an access signal generator. The access signal generator can be configured to modify a magnitude of a signal to generate a modified magnitude for the signal to access a resistive memory element associated with a word line and a subset of bit lines. The modified magnitude can be a function of the position of the resistive memory element in the cross-point array.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/150,521, filed Jan. 8, 2014, which is a continuation of and claimspriority to U.S. patent application Ser. No. 13/658,697, filed Oct. 23,2012, now issued as U.S. Pat. No. 8,654,565, which is a continuation ofand claims priority to U.S. patent application Ser. No. 13/425,247,filed Mar. 20, 2012, now issued as U.S. Pat. No. 8,305,796, which is adivision of and claims priority to U.S. patent application Ser. No.12/657,895, filed Jan. 29, 2010, now issued as U.S. Pat. No. 8,139,409,all of which are incorporated herein in their entireties. Thisapplication is also related to U.S. patent application Ser. No.11/095,026, filed Mar. 30, 2005, published as U.S. Pub. No.2006/0171200, and entitled “Memory Using Mixed Valence ConductiveOxides,” and with U.S. patent application Ser. No. 11/881,500, filedSep. 11, 2008, published as U.S. Pub. No. 2009/002797, now issued U.S.Pat. No. 7,701,791, and entitled “Low Read Current Architecture forMemory,” both of which are incorporated herein by reference for allpurposes.

FIELD OF THE INVENTION

Embodiments of the invention relate generally to semiconductors andmemory technology, and more particularly, to systems, integratedcircuits, and methods to generate access signals to facilitate memoryoperations in scaled arrays of memory elements, such as implemented inthird dimensional memory technology.

BACKGROUND

Scaling the dimensions of memory arrays and cells affect operationalcharacteristics of memory technologies. In some memory technologies, areduction in size of word lines or bit lines can increase theresistivity of those lines as the cross-sectional area of conductivepaths is reduced also. The increased resistance of word lines or bitlines may produce a reduction of voltage (e.g., voltage drops) alongthose lines, for example, as a function of the amount of memory cellsconducting current from the word lines or bit lines.

At least some conventional memory architectures, such as those includingdynamic random access memory (“DRAM”) cells and Flash memory cells,typically include gates as part of metal oxide semiconductor (“MOS”)transistors or structures. The gates operate to open and closeconductive paths between the word lines or bit lines and portions of thememory cells used as storage. When one of the conventional memory cellsis unselected, its gate is in an “off” mode of operation and conductsnegligible to no current. The gate structures used in conventionalmemory architectures buffer the conventional memory cells from theaffects of increased resistance of word lines or bit lines. Theabove-described memory architectures, while functional for theirspecific technologies, are not well suited to address the scaling ofmemory array and cell dimensions for other memory technologies.

It would be desirable to provide improved systems, integrated circuits,and methods that minimize one or more of the drawbacks associated withconventional techniques for facilitating memory operations in scaledmemory arrays and cells.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its various embodiments are more fully appreciated inconnection with the following detailed description taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 depicts an access signal generator in accordance with variousembodiments of the invention;

FIG. 2 depicts an access signal generator including a slice-rollingcontroller in accordance with various embodiments of the invention;

FIG. 3 is a diagram depicting an example of a structure for a slice inaccordance with embodiments of the invention;

FIG. 4 depicts an example of a word line voltage generator configured toaccess an example of a slice, according to various embodiments of theinvention;

FIG. 5 depicts a diagram of a target magnitude generator, according toat least some embodiments of the invention;

FIG. 5A depicts a block diagram representing the basic components of oneembodiment of a memory element;

FIG. 5B depicts a block diagram of the memory element of FIG. 5A in atwo-terminal memory cell;

FIG. 5C depicts a block diagram of the memory element of FIG. 5A in athree-terminal memory cell;

FIG. 6 depicts a diagram depicting another example of a target magnitudegenerator adapted to address disturb effects, according to someembodiments;

FIG. 7 depicts a diagram depicting how a disturb isolation circuit canoperate to address disturb effects, according to some embodiments;

FIG. 8 depicts a diagram depicting a target magnitude generator with aspecific implementation of a disturb isolation circuit, according tosome embodiments; and

FIG. 9 depicts an example of a cross-point array including multiplelayers of memory, according to various embodiments of the invention.

Like reference numerals refer to corresponding parts throughout theseveral views of the drawings. Note that most of the reference numeralsinclude one or two left-most digits that generally identify the figurethat first introduces that reference number.

DETAILED DESCRIPTION

Various embodiments or examples of the invention may be implemented innumerous ways, including as a system, a process, an apparatus, or aseries of program instructions on a computer readable medium such as acomputer readable storage medium or a computer network where the programinstructions are sent over optical, electronic, or wirelesscommunication links. In general, operations of disclosed processes maybe performed in an arbitrary order, unless otherwise provided in theclaims.

A detailed description of one or more examples is provided below alongwith accompanying figures. The detailed description is provided inconnection with such examples, but is not limited to any particularexample. The scope is limited only by the claims, and numerousalternatives, modifications, and equivalents are encompassed. Numerousspecific details are set forth in the following description in order toprovide a thorough understanding. These details are provided as examplesand the described techniques may be practiced according to the claimswithout some or all of the accompanying details. For clarity, technicalmaterial that is known in the technical fields related to the exampleshas not been described in detail to avoid unnecessarily obscuring thedescription.

U.S. patent application Ser. No. 11/095,026, filed Mar. 30, 2005,published as U.S. Pub. No. 20060171200, and entitled “Memory Using MixedValence Conductive Oxides,” is hereby incorporated by reference in itsentirety for all purposes and describes non-volatile third dimensionalmemory elements that may be arranged in a two-terminal, cross-pointmemory array. New memory structures are possible with the capability ofthis third dimensional memory array. In at least some embodiments, atwo-terminal memory element or memory cell can be configured to changeconductivity when exposed to an appropriate voltage drop across thetwo-terminals. The memory element can include an electrolytic tunnelbarrier and a mixed valence conductive oxide in some embodiments, aswell as multiple mixed valence conductive oxide structures in otherembodiments. A voltage drop across the electrolytic tunnel barrier cancause an electrical field within the mixed valence conductive oxide thatis strong enough to move oxygen ions out of a mixed valence conductiveoxide, according to some embodiments.

In some embodiments, an electrolytic tunnel barrier and one or moremixed valence conductive oxide structures (e.g., one or more layers of aconductive oxide material) do not need to operate in a siliconsubstrate, and, therefore, can be fabricated above circuitry being usedfor other purposes. That is, the active circuitry portion can befabricated front-end-of-the-line (FEOL) on a substrate (e.g., aSilicon—Si wafer or other semiconductor substrate) and one or morelayers of two-terminal cross-point memory arrays that include thenon-volatile memory elements can be fabricated back-end-of-the-line(BEOL) directly on top of the substrate and electrically coupled withthe active circuitry in the FEOL layer using an inter-level interconnectstructure also fabricated FEOL. Further, a two-terminal memory elementcan be arranged as a cross-point such that one terminal is electricallycoupled with an X-direction line (or an “X-line”) and the other terminalis electrically coupled with a Y-direction line (or a “Y-line”). A thirddimensional memory can include multiple memory elements verticallystacked upon one another, sometimes sharing X-direction and Y-directionlines in a layer of memory, and sometimes having isolated lines. When afirst write voltage, VW1, is applied across the memory element (e.g., byapplying ½ VW1 to the X-direction line and ½−VW1 to the Y-directionline), the memory element can switch to a low resistive state. When asecond write voltage, VW2, is applied across the memory element (e.g.,by applying ½ VW2 to the X-direction line and ½−VW2 to the Y-directionline), the memory element can switch to a high resistive state. Memoryelements using electrolytic tunnel barriers and mixed valence conductiveoxides can have VW1 opposite in polarity from VW2.

FIG. 5A shows an electrolytic tunnel barrier 505 and an ion reservoir511, two basic components of the memory element 501. FIG. 5B shows thememory element 501 between a top memory electrode 515 and a bottommemory electrode 520. The orientation of the memory element (i.e.,whether the electrolytic tunnel barrier 505 is near the top memoryelectrode 515 or the bottom memory electrode 520) may be important forprocessing considerations, including the necessity of seed layers andhow the tunnel barrier reacts with the ion reservoir 511 duringdeposition. FIG. 5C shows the memory element 501 oriented with theelectrolytic tunnel barrier 505 on the bottom in a three-terminaltransistor device, having a source memory element electrode 525, gatememory element electrode 530 and a drain memory element electrode 535.In such an orientation, the electrolytic tunnel barrier 505 could alsofunction as a gate oxide. Referring back to FIG. 5A, the electrolytictunnel barrier 505 will typically be between 10 and less than 50angstroms. If the electrolytic tunnel barrier 505 is much greater than50 angstroms, then the voltage that is required to create the electricfield necessary to move electrons through the memory element 501 viatunneling becomes too high for most electronic devices. Depending on theelectrolytic tunnel barrier 505 material, a preferred electrolytictunnel barrier 505 width might be between 15 and 40 angstroms forcircuits where rapid access times (on the order of tens of nanoseconds,typically below 100 ns) in small dimension devices (on the order ofhundreds of nanometers) are desired. Fundamentally, the electrolytictunnel barrier 505 is an electronic insulator and an ionic electrolyte.As used herein, an electrolyte is any medium that provides an iontransport mechanism between positive and negative electrodes. Materialssuitable for some embodiments include various metal oxides such asAl₂O₃, Ta₂O₅, HfO₂ and ZrO₂. Some oxides, such as zirconia might bepartially or fully stabilized with other oxides, such as CaO, MgO, orY₂O₃, or doped with materials such as scandium. The electrolytic tunnelbarrier 505 will typically be of very high quality, being as uniform aspossible to allow for predictability in the voltage required to obtain acurrent through the memory element 501. Although atomic layer depositionand plasma oxidation are examples of methods that can be used to createvery high quality tunnel barriers, the parameters of a particular systemwill dictate its fabrication options. Although tunnel barriers can beobtained by allowing a reactive metal to simply come in contact with anion reservoir 511, as described in PCT Patent Application No.PCT/US04/13836, filed May 3, 2004, already incorporated herein byreference, such barriers may be lacking in uniformity, which may beimportant in some embodiments. Accordingly, in a preferred embodiment ofthe invention the tunnel barrier does not significantly react with theion reservoir 511 during fabrication. With standard designs, theelectric field at the tunnel barrier 505 is typically high enough topromote tunneling at thicknesses between 10 and 50 angstroms. Theelectric field is typically higher than at other points in the memoryelement 501 because of the relatively high serial electronic resistanceof the electrolytic tunnel barrier 505. The high electric field of theelectrolytic tunnel barrier 505 also penetrates into the ion reservoir511 at least one Debye length. The Debye length can be defined as thedistance which a local electric field affects distribution of freecharge carriers. At an appropriate polarity, the electric field withinthe ion reservoir 511 causes ions (which can be positively or negativelycharged) to move from the ion reservoir 511 through the electrolytictunnel barrier 505, which is an ionic electrolyte. The ion reservoir 511is a material that is conductive enough to allow current to flow and hasmobile ions. The ion reservoir 511 can be, for example, an oxygenreservoir with mobile oxygen ions. Oxygen ions are negative in charge,and will flow in the direction opposite of current. Each memory plugcontains layers of materials that may be desirable for fabrication orfunctionality. For example, a non-ohmic characteristic that exhibit avery high resistance regime for a certain range of voltages (V_(NO−) toV_(NO+)) and a very low resistance regime for voltages above and belowthat range might be desirable. In a cross point array, a non-ohmiccharacteristic could prevent leakage during reads and writes if half ofboth voltages were within the range of voltages V_(NO−) to V_(NO+). Ifeach conductive array line carried ½ V_(W), the current path would bethe memory plug at the intersection of the two conductive array linesthat each carried ½ V_(W). The other memory plugs would exhibit suchhigh resistances from the non-ohmic characteristic that current wouldnot flow through the half-selected plugs.

FIG. 1 depicts an access signal generator in accordance with variousembodiments of the invention. In this example, an access signalgenerator 102 is coupled to a line driver 104, which, in turn, iscoupled via a number of access lines 116 to memory cells in an array110. In some embodiments, array 110 is subdivided into slices 112, suchas slice (“0”) 112 a, slice (“1”) 112 b, and slice (“N”) 112 n, eachslice representing a group of memory cells. A cache 150, or equivalentmemory structure, is shown to receive the data read from slices 112. Thedata read into cache 150 (or any type of read buffer) can be in the formof a page or any other of unit of data. Access signal generator 102 isconfigured to modify a magnitude of signal to generate a modifiedmagnitude to access a memory element in a memory cell associated with anaccess line 116 and one of slices 112, the modified magnitude being afunction of the location of the memory element in array 110. In someembodiments, access signal generator 102 includes a positionalcharacteristic adjuster 103 configured to determine the magnitude of thesignal as a function of a distance 119 between the position of a memoryelement and access signal generator 102. In some embodiments, a memoryelement (“M”) 170 is a resistive memory element configured to maintain aresistive state representative of a data stored therein. According to atleast one embodiment, a reference element 160 can be disposed in aregion local to memory element 170 to resolve the logical state of datastored in memory element 170. Optionally, access signal generator 102can be configured to determine (e.g., modify) the magnitude based on anindicator signal via a feedback path 109, the indicator signalrepresenting the magnitude of the signal at a location in array 110.

In view of the foregoing, the structures and/or functionalities ofaccess signal generator 102 can provide for sufficient signal magnitudesto reliably access values (e.g., parametric values of resistances,currents, voltages, etc.) representing data stored in memory element 170over various scaled dimensions of, for example, access lines 116, memoryelements 170, slices 112, and/or array 110. In some embodiments, thevarious structures and/or functionalities of access signal generator 102described herein can be configured to adjust the magnitude of the signalfor accessing memory element 170 to compensate for one or more voltagedrops associated with one or more other resistive memory elements.Therefore, access signal generator 102 can be configured to adjust themagnitude of the signal to compensate for a deviation in the magnitudefrom a target magnitude (e.g. such as a value for a target readvoltage). To illustrate, consider that access signal generator 102 isconfigured to cause line driver 104 to generate a read voltage foraccessing memory element 170 to read information stored therein, theread voltage being determined (e.g., being adjusted) based on thelocation of memory element 170 to compensate for one or more voltagedrops over one of access lines 116 due to, for example, other unselectedmemory elements. In other memory operations, such as in a writeoperation or erase operation, line driver 104 and line driver 115 canindividually or collectively determine (e.g., adjust) a write or erasevoltage to compensate for the one or more voltage drops, according tosome embodiments. An adjusted read voltage is configured to apply atarget read voltage to a memory element 170 regardless of its positionalong access line 116. In various embodiments, a reference element 160is disposed locally adjacent to memory element 170 (e.g., in slice 112n) and is configured to provide a reference signal based on the modifiedmagnitude of an access signal. In various embodiments, the term “accesssignal” can refer to any type of signal (e.g., a voltage signal, acurrent signal, or any other signal) for accessing one or more memorycells in a memory operation, such as a read operation, a writeoperation, or an erase operation. The reference signal during readoperations can be based on a position-dependent magnitude for a readvoltage. In other memory operations, such as in a write operation or inan erase operation, the reference signal can be based on theposition-dependent magnitude of a write voltage or an erase voltage,according to some embodiments. Also, a locally adjacent referenceelement 160 can enhance a sensing margin. Further, an indicator signalcan be generated locally in a slice that includes the memory element tobe accessed. For example, an indicator signal can be generated locallyin slice 112 n to access memory element 170, according to someembodiments. The indicator signal is configured to convey informationrepresentative of the magnitude of the access signal via one or morefeedback paths 109 at a position of interest in array 110, such as at ornear memory element 170. In read operations, a locally-generatedindicator signal can be based on the position-dependent magnitude of theread voltage. In other memory operations, such as in a write operationor in an erase operation, the locally-generated indicator signal can bebased on the position-dependent magnitude of a write voltage or an erasevoltage, according to some embodiments.

To illustrate operation of access signal generator 102, consider thefollowing example in which access signal generator 102 is configured togenerate read voltage signals to read data from array 110, and themagnitude of the read voltages vary (e.g., increase) as a function ofposition 119 between position 0 and position N at which a memory elementis accessed (e.g., for reading, writing, erasing, or for performingother memory-related operations). For an address 111, access signalgenerator 102 can identify one of access lines 116 and the associatedmemory elements to be access during a memory cell access operation, suchas a read operation, a write operation, an erase operation, etc. Notethat in some embodiments, the terms “position” and “distance” can beinterchangeable. Positional characteristic adjuster 103 is configured todetermine a magnitude of a read voltage as a characteristic of a signalas a function of position as depicted in relationship 106. For example,for positional characteristic adjuster 103 can modify the read voltagegenerated by line driver 104 to form a modified magnitude (“Vrd0”) 108 aso that at a first memory element, such as in slice 112 a, the readvoltage has a magnitude approximated to a target read voltage magnitude(“Vtarg”) 108 d. Thus, access signal generator 102 can generate modifiedmagnitude (“Vrd0”) 108 a to compensate for a differential 108 e (e.g.,due to one or more voltage drops) along an access line 116 to apply atarget magnitude 108 d at or near the memory element being accessed.Similarly, positional characteristic adjuster 103 can modify the readvoltages generated by line driver 104 to form modified magnitudes(“Vrd1”) 108 b and (“VrdN”) 108 c that are applied to one of accesslines 116 so that the read voltages having magnitudes approximated to atarget read voltage magnitude (“Vtarg”) 108 d can be applied on anaccess line 116 at or near a second memory element and a third memoryelement, respectively, disposed in slices 112 b and 112 n. In someembodiments, access signal generator 102 and line driver 104 cooperateto apply different modified magnitudes of an access signal to readdifferent portions of read buffer. For example, as access signalgenerator 102 generates modified magnitudes 108 a, 108 b, and 108 c foran access signal, the data can be read from slices 112 a, 112 b, and 112n into respective as portions 114 a, 114 b, and 114 n of a read buffer,such as cache 150.

In at least one embodiment, access signal generator 102 includes adisturb isolator 105 configured to isolate or reduce the effects ofphenomena that can cause a “disturb.” The term disturb generally refersto the disturb effects, such as the electrical and/or electromagneticcoupling (or otherwise), on neighboring memory elements or cells notselected for accessing when other memory cells are accessed (e.g., fordata operations on the accessed memory cell(s)). For example,application of read voltages to one of access lines 116 one or moretimes to read selected memory elements can affect or otherwise disturbthe operability of unselected memory elements. Therefore, disturbisolator 105 can be configured to condition an access signal to accessselected memory elements while isolating or reducing the disturb effectson unselected memory elements.

Memory element 170 can be selected by activating a line extending fromline driver 115 and activating (e.g., applying a read voltage with amodified magnitude) one of access lines 116. In some embodiments, linedriver 115 is configured as a Y-line driver and/or encoder to driveY-lines (e.g., arranged in columns or bit lines) in a cross-point array,whereas line driver 104 is configured as an X-line driver and/or encoderto drive X-lines (e.g., arranged in rows or word lines). Note that invarious embodiments, access signal generator 102 can be implemented ineither X-lines or Y-lines, or both. Further, access signal generator 102can be configured to generate modified magnitudes for access signalsthat are used to program or erase memory element 170, and, as such, cangenerate modified programming voltage magnitudes and/or modified erasingvoltage magnitudes. In a specific implementation, a slice can includeany number of Y-lines. For example, a slice can include 256 to 2,048Y-lines, or more. In some embodiments, the structures and/orfunctionalities (or portions thereof) can be implemented in line driver104 or line driver 115. Note that the terms “write”, “program”, and“erase” can be used interchangeably, according to some embodiments. Forexample, a write operation can comprise a programming operation or anerase operation on one or more memory elements and different magnitudesand polarities of a write voltage can be used to perform the program orerase operations.

FIG. 2 depicts an access signal generator including a slice-rollingcontroller in accordance with various embodiments of the invention. Asshown in diagram 200, an access signal generator 202 is configured tocontrol a word line driver 260 to generate word line voltages withmodified magnitudes, with word line driver 260 being disposedelectrically between two different arrays, such as array 230 and array240 (or between portions of arrays). Array 230 includes slice (“1”) 232a, slice (“3”) 232 b, and slice (“N”) 232N, whereas array 240 includesslice (“0”) 242 a, slice (“2”) 242 b, and slice (“N−1”) 242 n. In atleast one embodiment, slice-rolling controller 204 is configured to“roll” thorough slices 232 and 242 to apply modified magnitudes of oneor more access signals sequentially to the slices. For example,slice-rolling controller 204 can generate and apply a first modifiedmagnitude of a read voltage to a group of memory elements associatedwith a group of bit lines (e.g., in slice 242 a) during a first intervalof time, and can apply a second modified magnitude of a read voltage toanother group of memory elements associated with another group of bitlines (e.g., in slice 242 b) during a second interval of time. Further,slice-rolling controller 204 can be configured to apply the samemagnitude of an access signal to different positions in arrays 230 and240, simultaneously or during different periods of time. In someexamples, the different positions can be substantially equidistant fromaccess signal generator 202. In a specific implementation, slice-rollingcontroller 204 can be configured further to increase the value of themodified magnitude as the distance increases between the differentpositions and access signal generator 202.

To illustrate, consider that slice-rolling controller 204 is configuredto generate modified magnitudes as depicted in relationships 210 and220. For example, slice-rolling controller 204 can be configured togenerate magnitude (“V0”) 222 a for transmission to a memory element ata position “0” in slice 242 a, and to generate magnitude (“V1”) 212 afor transmission to a memory element at a position “1” in slice 232 a.Note that magnitudes 222 a and 212 a can be the same (or substantiallythe same). Subsequently, slice-rolling controller 204 can be configuredto generate magnitude (“V2”) 222 b for transmission to a memory elementat a position “2” in slice 242 b, and to generate magnitude (“V3”) 212 bfor transmission to a memory element at a position “3” in slice 232 b.Note that magnitudes 222 b and 212 b can be the same (or substantiallythe same), and can be greater than magnitudes 222 a and 212 a. In oneembodiment, slice-rolling controller 204 can be configured to applymagnitudes 212 a and 222 a to respective slices 232 a and 242 asimultaneously (e.g., to effect simultaneous reads of memory elements inslices 232 a and 242 a), with subsequent simultaneous application ofmagnitudes 212 b and 222 b to respective slices 232 a and 242 a. Inanother embodiment, slice-rolling controller 204 can be configured toapply magnitudes 212 a and 222 a to respective slices 232 a and 242 a atdifferent intervals of time (e.g., to effect staggered reads of memoryelements in slices 232 a and 242 a), such as depicted in diagram 200. Asshown, slice-rolling controller 204 first applies a first modifiedmagnitude to slice 242 a during a first time interval, and then appliesthe first modified magnitude to slice 232 a during a second timeinterval. Next, slice-rolling controller 204 then applies a secondmodified magnitude to slice 232 b during a third time interval, and thenapplies the second modified magnitude to slice 242 b during a fourthtime interval. Slice-rolling controller 204 can operate in accordancewith other schemes and is not limited to the above-described examples.

FIG. 3 depicts a diagram illustrating an example of a structure for aslice in accordance with embodiments of the invention. Diagram 300depicts a slice (“0”) 312 including a number of access lines 330 and 332arranged in one orientation, and another number of access lines 314, 316a, and 316 n arranged in another orientation (e.g., orthogonal to lines330 and 332). Also shown, slice 312 is coupled to sensing circuitry,including sense amplifier (“SA”) 340 and sense amplifier (“SA”) 342. Insome embodiments, access lines 330 and 332 are word lines, access line314 is a reference bit line 314, and access lines 316 a and 316 n arebit lines arranged as memory columns (i.e., bit lines associated withmemory elements to store information). Reference cells (“R”) 322 arecoupled at one side to reference bit line 314 and at another side to oneof word lines 330 and 332. Memory cells (“M”) 324 are coupled at oneside to one of bit lines 316 a to 316 n and at another side to one ofword lines 330 and 332.

Reference cell 322 include a reference memory element 320, which isformed to have the same (or approximately the same) structureand/functionality as a memory element 328 that constitutes the storagestructure in memory cell 324. As reference memory element 320 is formedadjacent to memory elements 328 a and 328 b, they are more likely to beformed more similarly than if reference memory element 320 is formedexternal to either slice 312 or to an array. Reference memory element320 is configured to generate a reference signal in association with themodified magnitude of the signal to apply a magnitude approximate to atarget magnitude (e.g., a target read voltage). To illustrate operationof reference cell 322, consider an example in which a read voltagemagnitude, Vrd, is applied to word line 332 to read data from memoryelement 328 b, and word lines 330 are unselected (e.g., word lines 330are set to ground). In embodiments in which memory element 328 b is aresistive state memory element, bit line 316 n is configured to transmita read current representative of a resistant state associated with alogic value stored in memory cell 324 b. The read current 370 and thereference signal 360 are provided via bit line 316 n and reference bitline 314, respectively, to sense amplifier 342, which, in turn, comparesthe read current to the reference signal to determine the logic value.

Note that bit line 316 a in the above example is configured to placememory element 328 a in an inactive state. In some instances, memoryelement 328 a is a resistive state memory element. When unselected,memory element 328 a can, in some applications, provide a conductionpath for current, which produces a voltage drop. Similar voltage dropscan exist between bit line 316 n and an access signal generator 301along word line 322, whereby the voltage drops associated with word line322 can aggregate to produce a collective voltage drop or differentialfor which access signal generator 301 is configured to address bymodifying the magnitude of the read voltage. In some embodiments, wordline 332 (as well as word lines 330) can be referred to as a “gateless”word line (or a gateless array line) as memory cells 324 a and 324 b mayomit gate-like mechanisms, such as a transistor gate or a MOStransistor, that otherwise operate as open circuits. In one embodiment,reference resistive memory element 320 is disposed in a portion of across-point array (e.g., in slice 312) that includes memory elements 328a and 328 b, both of which can be resistive memory elements. In someembodiments, reference bit line 314 can be disposed in between equalquantities of bit lines between bit lines 316 a and 316 n (e.g., in themiddle of slice 312). By forming reference bit line 314 in the middle ofslice 312, reference memory elements can receive a read voltagemagnitude that approximates an average of the actual read voltagemagnitudes over word line 322 within slice 312. In particular, referencebit line 314 being disposed in the middle of slice 312 has an equivalentamount of voltage drops on either side of it and experiences half thenumber of voltage drops between points 380 and 382 that memory element328 b experiences. In alternate embodiments, multiple reference bitlines 314 can used (not shown), whereby the reference memory elementsfor each of the multiple bit lines 314 provide different referencesignal magnitudes. For example, three reference bit lines 314 can eachinclude a group of different reference memory elements. A first group ofreference memory elements can be configured to generate a referencesignal having a magnitude representative of a programmed state, whereasa second group can be configured to generate another reference signalhaving a magnitude representative of an erased state. Yet a third groupof reference memory elements can be configured to generate a referencesignal having a magnitude in a range of magnitudes defined by magnitudesrepresentative of the erased state and the programmed state. Note thatany number of reference bit lines 314 can be implemented in slice 312 atany location therein.

FIG. 4 depicts an example of a word line voltage generator configured toaccess an example of a slice, according to various embodiments of theinvention. Diagram 400 depicts a word line voltage generator 402 coupledvia a word line driver 408 to an array 410 including any number ofslices, such as slice (“0”) 412. Also shown is a multiplexer (“MUX”) 450is coupled between slice 412 and word line voltage generator 402, and isconfigured to multiplex signals from slice 412 and from other slices notshown. In the example shown, slice 412 includes word lines 430 and 432extending into array 410 from word line driver 408. Slice 412 alsoincludes one or more bit lines configured as indicator columns 413, oneor more bit lines configured as reference columns 414, and a number ofbit lines configured as memory columns 416. Similar to reference bitlines 314 and bit lines 316 of FIG. 3, one or more reference columns 414are configured to provide reference signals generated by referencememory elements (“R”) 422, and memory columns 416 are each configured toprovide read current signals generated by memory elements (“M”) 424, theread current signals representing stored data. Sense amplifier (“SA”)440 is coupled to one or more reference columns 414 and memory columns416 to generate a Data signal(s) representative of data read from memoryelements 424. Further, diagram 400 depicts indicator memory cells 421coupled between indicator column 413 and respective word lines 430 and432.

Indicator column 413 is configured to provide for real-time (or nearreal-time) word line voltage sensing and to convey indicator signals 460generated by indicator memory elements (“ ”) 420. As a word linetracking reference, the indicator signals are representative of adetected magnitude at one of word lines 430 and 432. The detectedmagnitude can be sampled at or near a position of memory element 424,which can be a resistance state memory element according to someembodiments. Also, the detected magnitude can represent a magnitude ofvoltage on word line 432, for example, at a distance from word linedriver 408 in which voltage drops reduce an amount of a read voltageapplied to word line 432. In some embodiments, indicator memory elementscan be configured to be in an erased state.

Word line voltage generator 402 is configured to generate a modifiedmagnitude for a read voltage signal responsive, at least in part, toindicator signal 460. The modified magnitude is configured to compensatefor the voltage drops between word line driver 408 and a position alongany of word lines 430 and 432. Word line voltage generator 402 caninclude a target magnitude generator 406 and a positional voltageadjuster 403. Target magnitude generator 406 can be configured togenerate a target magnitude for the read voltage signal to be appliedvia word line 432 to memory cell 424. Positional voltage adjuster 403 iscoupled to target voltage generator 406 to receive a read voltage signalhaving a magnitude equivalent or approximate to the target magnitude,and, as shown, positional voltage adjuster 403 can be coupled to MUX 450to receive indicator signal 460. Positional voltage adjuster 403operates to compare the detected magnitude derived from indictor signal460 to the target magnitude produced by target magnitude generator 406.Positional voltage adjuster 403 further operates to determine a voltageerror (i.e., a deviation from the target magnitude) and to adjust themagnitude of the read voltage to form a modified magnitude.

FIG. 5 depicts a diagram of a target magnitude generator, according toat least some embodiments of the invention. In the example shown indiagram 500, target magnitude generator 506 includes a dummy array 510configured to receive a voltage (“Vrd1 (IN)”) at terminal 560 and togenerate a read voltage (“Vrd2 (IN)”) having a target magnitude atterminal 562. Dummy array 510 includes a dummy word line 532 coupled toterminal 560 and to a first terminal of a dummy cell 512, which includesa dummy memory element (“D”) 514. Dummy array 510 also includes a numberof word lines 530 a, 530 b, and 530 c that can be grounded or otherwiseset to a magnitude representative of an unselected state. For example,the magnitude representative of an unselected state can be a voltagelevel similar to those voltage levels applied to unselected memory cellsin the array (e.g., array 110 of FIG. 1). Dummy array 510 furtherincludes a number of bit lines 518 that can be grounded, and a dummy bitline 516 coupled to terminal 562 and a second terminal of memory element514. Dummy array 510 is configured to condition the voltage applied toterminal 560 to generate a target read voltage magnitude at terminal562. Note that dummy array 510 can be implemented to provide a targetwrite voltage and/or a target erase voltage magnitude for other memoryoperations, according to some embodiments. In some cases, dummy array510 can be configured to provide one or more target voltage magnitudes(e.g., one or more of a target read voltage magnitude, a target writevoltage magnitude and a target erase voltage magnitude).

Note that in some instances, dummy array 510 can be formed external toeither one or more slices or an array, such as a cross-point array.According to some embodiments, dummy memory element 514 is a dummyresistive state memory element configured to emulate operation of anindicator resistive memory element. In some embodiments, dummy memoryelement 514 can be configured to be in an erased state. Similarly, dummybit line 516 can be configured to emulate operation of an indicatorcolumn disposed in a slice with an array. According to a specificembodiment, the voltage (“Vrd1 (IN)”), or variations thereof, can beapplied to each of dummy word line 532, word line 530 a, word line 530b, and word line 530 c at different points in times. Optionally, each ofdummy word line 532, word line 530 a, word line 530 b, and word line 530c can be used to access different slices or groups of bit lines in thearray. For example, dummy element 514 can be used for one access line116 of FIG. 1, and dummy element 560 a (or dummy elements 560 b or 560c) can be used to access memory cells, for example, in other accesslines 116. In some instances, using different dummy elements 514, 560 a,560 b, and 560 c can experience less usage (e.g., operational stress)than if one of dummy elements 514, 560 a, 560 b, and 560 c were used formultiple groups of slices. Note that while FIG. 5 depicts four (4) dummyelements, the various embodiments are not limited to any specific numberof dummy elements, and, further, are not limited to any specific numberof slices that each of dummy elements 514, 560 a, 560 b, and 560 c canrelate.

FIG. 6 depicts a diagram depicting another example of a target magnitudegenerator adapted to address disturb effects, according to someembodiments. Diagram 600 depicts a target magnitude generator 606including a disturb isolation circuit 660 and a dummy array 610, whichcan have a structure and/or function similar to dummy array 510 of FIG.5. Disturb isolation circuit 660 can be configured to isolate or reducedisturb effects or other similar phenomena, and can compensate fordisturbances of the magnitude of a read voltage signal associated with aparticular word line. An example of a disturb effect is caused byrepeated applications of read voltages (e.g., a target read magnitude),whereby the disturb effect can affect data retention in unselectedmemory elements or can affect operation of a reference memory element.In one embodiment, disturb isolation circuit 660 is configured to reducethe target read magnitude and/or an amount of voltage swings on the wordlines to reduce or negate disturb effects on, for example, unselectedmemory elements. In a specific embodiment, disturb isolation circuit 660can be configured to reduce the magnitude of the read voltage signalwhile maintaining an access current equivalent to that associate with afirst magnitude of the read voltage signal.

FIG. 7 depicts a diagram depicting how a disturb isolation circuit canoperate to address disturb effects, according to some embodiments.Diagram 700 depicts a non-linear relationship to a read voltage appliedto a memory element, such as a resistive state memory element, and acorresponding read current to access the memory element. As shown, adisturb isolation circuit can operate to reduce a target read voltagemagnitude, Vrd, to approximate one-half the target read voltagemagnitude, Vrd/2. The disturb isolation circuit can compensate for thereduction in read current when reducing a read current, Ird, from point702 to a read current, Ird@Vrd/2, at point 704. Specifically, thedisturb isolation circuit boosts the read current associated with point704 by an amount (“Idiff”) 750 to provide an amount of current, Ird,when the target read voltage magnitude is halved.

FIG. 8 depicts a diagram depicting a target magnitude generator with aspecific implementation of a disturb isolation circuit, according tosome embodiments. Target magnitude generator 806 includes a disturbisolation circuit 810 composed of a current enhancement array 820.Further, target magnitude generator 806 can include a current selector840, a multiplexer (“MUX”) 842, and a dummy array 850. Currentenhancement array 820 can include resistive memory elements thatconfigured to generate an amount of current at a portion of the targetmagnitude of the signal (i.e., the portion represents a reduce targetread voltage magnitude). The amount of current generated by currentenhancement array 820 can be equivalent to a current generated by aresistive memory element with the target magnitude of the signal.Current enhancement array 820 can include a number of resistive memoryelements coupled in parallel, the resistive memory elements beingdepicted as part of dummy cells (“D”) 822.

Current selector 840 is configured to control MUX 842 to select a subsetof bit lines 830 from which to receive read current amount from aparticular number of resistive memory element that can generate anenhanced or boosted read current. In some embodiments, current selector840 can generate an enhanced current to ensure thatresistive-capacitance (“RC”) characteristics (e.g., such as duringcurrent charging of a capacitive load) for a bit line are equivalent tothat for a single dummy memory element generating a full target readvoltage magnitude. Dummy array 850 can be configured for loadingpurposes to emulate operation of indicator memory elements and/or memoryelements in a slice.

FIG. 9 depicts an example of a cross-point array of memory cellsincluding multiple layers of memory, according to various embodiments ofthe invention. Although multiple layers of memory are depicted, thecross-point array can include only a single layer of memory. In thisexample, diagram 900 depicts a portion 910 of a cross-point array thatforms one of multiple layers of memory 950, which are formed on or abovea substrate 990 that includes a logic layer 970 having active circuitryoperative to perform data operations on the one or more memory layers950. The substrate 990 can be a silicon (Si) wafer upon which circuitryin the logic layer 970 (e.g., CMOS circuitry) is fabricated as part of afront-end-of-the-line (FEOL) fabrication process. An inter-layerinterconnect structure (not shown) fabricated as part of the FEOLprocessing can include electrically conductive interconnect structures(e.g., vias, throughs, plugs, contacts, or the like) configured toelectrically couple the circuitry in the logic layer 970 with one ormore memory layers 950 that are fabricated directly on top of and incontact with the substrate 990. Subsequently, the one or more layers ofmemory 950 can be fabricated directly on top of an upper surface 990 s(e.g., along the +Z axis) of the substrate 990 as part of aback-end-of-the-line (BEOL) fabrication process tailored for fabricatingnon-volatile two-terminal cross-point memory arrays. The upper surface990 s can be an upper surface of the aforementioned FEOL inter-layerinterconnect structure. If multiple layers of BEOL memory 950 arefabricated, the multiple layers are vertically stacked upon one anotheralong the +Z axis. After FEOL and BEOL processing are completed, thesilicon wafer can be cingulated into individual silicon die 999, witheach die 999 being an integrated circuit having a FEOL portion 990 withactive circuitry 970 fabricated thereon and a BEOL memory portion 950(e.g., three vertically stacked layers of memory) that are a unitarywhole, that is, the BEOL portion is grown directly on top of the FEOLportion to form a single die 999 that can be mounted in a suitablepackage (not shown) for an IC and wire bonded or the like toelectrically couple with pins on the package. As shown, portion 910 ofthe cross-point array includes conductive X-lines 930 and 932, and ispartitioned into slices 912 a and 912 b, each including a subset ofconductive Y-lines, including Y-lines 920, 922, and 924. In oneembodiment, a slice includes at least one Y-line 920 that is configuredas an indicator column and a number of Y-lines 924 that are configuredas memory columns. In another embodiment, a slice includes at least oneY-line 922 that is configured as a reference column and a number ofY-lines 924 that are configured as memory columns. According to variousembodiments, indicator columns and/or reference column are optional andneed not be implemented in every slice. For example, indicator columnsand/or reference column can be implemented in every “N” number of slicesalong conductive X-lines 930 and 932, where N can represent two or more.As such, reference signals and/or indicator signals can be generated formultiple slices. Note that while FIG. 9 depicts slices 912 a and 912 bbeing oriented in a layer of memory coincident with an X-Y plane, slices912 a and 912 b are not limited to the X-Y plane and can oriented in theY-Z and X-Z planes, according to other embodiments. Further, each ofslices 912 a and 912 b can include multiple sets of Y-lines 920, 922,and 924, with each set being disposed at different X-Y planes along theZ-axis relative to the logic layer 970.

Word line (“WL”) voltage generator 972 is configured to receive anindicator signal via path 962 from an indicator memory element 940, and,in response, generate a read voltage signal having a modified magnitude,Vrd, for transmission via path 960 to word line 932. The read voltagesignal is applied via X-line 932 to terminals of a reference memoryelement 942 and a memory element 944 for generating a reference signalon Y-line 922 and a read current signal on Y-line 924, respectively. Thereference signal and the read current signal traverse path 964 and path966, respectively, for delivery to sensing circuits 974. In someembodiments, a dummy array 952 and/or a current enhancement (“CE”) array954 include memory elements distributed in any layer within the multiplelayers of memory 950. Note that while FIG. 9 depicts dummy array 952 andcurrent enhancement (“CE”) array 954 being disposed on different layersof memory, they need not be so limited. Thus, dummy array 952 andcurrent enhancement array 954 (or portions thereof) can be disposed onthe same layer of memory or can be distributed over multiple layers ofmemory. In at least one embodiment, dummy array 952 and currentenhancement (“CE”) array 954 are disposed in the same layer of memory asare the slices to which arrays 952 and 954 relate.

In some embodiments, a memory element describe in this figure or anyfigure herein can be implemented as a resistive memory element 902 thatincludes a structure 904 including an electrolytic insulator (“EI”)disposed on a structure 909 including one or more layers of a conductiveoxide material, such as a conductive metal oxide-based (“CMO-based”)material, for example. Memory element 902 further includes two terminals(not shown). In various embodiments, CMO material 909 can include but isnot limited to a material selected from one or more the following:PrCaMnO_(X) (PCMO), LaNiO_(X) (LNO), SrRuO_(X) (SRO), LaSrCrO_(X)(LSCrO), LaCaMnO_(X) (LCMO), LaSrCaMnO_(X) (LSCMO), LaSrMnO_(X) (LSMO),LaSrCoO_(X) (LSCoO), and LaSrFeO_(X) (LSFeO), where x is nominally 3 forperovskites. In various embodiments, electrolytic insulator 904 caninclude but is not limited to a material for implementing a tunnelbarrier layer, the material being selected from one or more of thefollowing: rare earth oxides, rare earth metal oxides, yttria-stabilizedzirconium (YSZ), zirconia (ZrO_(X)), yttrium oxide (YO_(X)), erbiumoxide (ErO_(X)), gadolinium oxide (GdO_(X)), lanthanum aluminum oxide(LaAlO_(X)), and hafnium oxide (HfO_(X)), and equivalent materials.Typically, the electrolytic insulator 904 comprises a thin film layerhaving a thickness of approximately less than 50 Å (e.g., in a rangefrom about 10 Å to about 35 Å).

The various embodiments of the invention can be implemented in numerousways, including as a system, a process, an apparatus, or a series ofprogram instructions on a computer readable medium such as a computerreadable storage medium or a computer network where the programinstructions are sent over optical or electronic communication links. Ingeneral, the steps of disclosed processes can be performed in anarbitrary order, unless otherwise provided in the claims.

The foregoing description, for purposes of explanation, uses specificnomenclature to provide a thorough understanding of the invention.However, it will be apparent to one skilled in the art that specificdetails are not required in order to practice the invention. In fact,this description should not be read to limit any feature or aspect ofthe present invention to any embodiment; rather features and aspects ofone embodiment can readily be interchanged with other embodiments.Notably, not every benefit described herein need be realized by eachembodiment of the present invention; rather any specific embodiment canprovide one or more of the advantages discussed above. In the claims,elements and/or operations do not imply any particular order ofoperation, unless explicitly stated in the claims. It is intended thatthe following claims and their equivalents define the scope of theinvention.

The invention claimed is:
 1. A method, comprising: identifying aselected two-terminal resistive memory element in an array including aplurality of two-terminal memory elements, each two-terminal resistivememory element being configured to retain stored data in an absence ofelectrical power and being serially-coupled to a word line and acorresponding bit line; generating an access signal having a magnitudebased at least in part on a position of the selected two-terminalresistive memory element in the array; and accessing the selectedtwo-terminal resistive memory element in response to the access signal.2. The method of claim 1, wherein the generating the access signalfurther comprises: generating the access signal using a signalgenerator; and modifying the magnitude of the access signal as afunction of a distance between the position of the selected two-terminalresistive memory element in the array and the signal generator.
 3. Themethod of claim 1, wherein the generating the access signal furthercomprises modifying the magnitude of the access signal to compensate fora voltage drop across another two-terminal resistive memory elementelectrically coupled with the word line and electrically coupled with adifferent bit line as the selected two-terminal resistive memoryelement.
 4. The method of claim 1, wherein the generating the accesssignal further comprises modifying the magnitude of the access signal tocompensate for a deviation in the magnitude of the access signal from atarget magnitude.
 5. The method of claim 1, wherein the generating theaccess signal further comprises modifying a read voltage magnitude ofthe access signal to enable a read operation on the selectedtwo-terminal resistive memory element.
 6. The method of claim 1, whereinthe generating the access signal further comprises modifying a writevoltage magnitude of the access signal to enable a write operation onthe selected two-terminal resistive memory element.
 7. The method ofclaim 1, wherein the generating the access signal further comprisesmodifying an erase voltage magnitude of the access signal to enable anerase operation on the selected two-terminal resistive memory element.8. The method of claim 1, wherein the generating the access signalfurther comprises modifying a program voltage magnitude of the accesssignal to enable a program operation on the selected two-terminalresistive memory element.
 9. The method of claim 1, further comprising:locating a two-terminal reference resistive memory element in a portionof the array including the selected two-terminal resistive memoryelement; and generating a reference signal based at least in part onelectrically coupling the two-terminal reference resistive memoryelement to a reference bit line and a word line corresponding to thetwo-terminal reference resistive memory element.
 10. The method of claim1, locating a two-terminal indicator resistive memory element in aportion of the array at or near the selected two-terminal resistivememory element; and generating an indicator signal representing adetected magnitude of the access signal at or near the position of theselected two-terminal resistive memory element.
 11. The method of claim1, further comprising: determining a target magnitude for the accesssignal; and modifying the magnitude of the access signal based at leastin part on the target magnitude.
 12. A method, comprising: extending atleast two slices of two-terminal resistive memory elements (MEs) in atleast one of XY, YZ, or XZ planes of a three dimensional array, each MEbeing configured to retain stored data in an absence of electricalpower; identifying a selected ME in the array; generating an accesssignal having a magnitude based at least in part on a position of theselected ME in the array; and accessing the selected ME in response tothe access signal.
 13. The method of claim 12, further comprisingadjusting the magnitude of the access signal as a function of a distancebetween the position of the selected ME and an access signal generator.14. The method of claim 12, further comprising adjusting the magnitudeof the access signal to compensate for a deviation from a targetmagnitude.
 15. The method of claim 12, further comprising: generating areference signal corresponding to a two-terminal reference resistivememory element (RME) electrically coupled with a reference bit line anda reference word line based at least in part on the access signal, theRME being disposed on a same slice that includes the selected ME. 16.The method of claim 12, further comprising: generating an indicatorsignal corresponding to a two-terminal indicator resistive memoryelement (IME) electrically coupled with an indicator bit line and anindicator word line based at least in part on detecting the magnitude ofthe access signal, the IME being disposed on a same slice that includesthe selected ME.
 17. The method of claim 16, further comprisingemulating operation of the IME using a dummy memory element (DME).
 18. Amethod for accessing memory cells in a cross-point array, comprising:identifying a two-terminal resistive memory element for access; andgenerating a voltage level as a function of a distance to thetwo-terminal resistive memory element; wherein the voltage level is aread voltage that generates an amount of current through thetwo-terminal resistive memory element at the distance.
 19. The method ofclaim 18, further comprising: detecting a local voltage level adjacentto one of the terminals of the two-terminal resistive memory element;and adjusting the local voltage level to approximate a target voltagelevel.
 20. The method of claim 18, further comprising: detecting a localvoltage level adjacent to one of the terminals of the two-terminalresistive memory element; and adjusting the voltage level so that thelocal voltage level approximates a target voltage level.